MDL-31560 Messages: Added support for noreply and support users to send message

message_send api can use core_user class to get noreply or support user
and then send message depending on the requirement.
MDL-31560 Messages: Replaced generate_email_supportuser with core_user::get_support_user()
MDL-31560 Messages: Updated NOREPLY_USER and SUPPORT_USER id's to avoid conflicts